1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying machine, or a facsimile machine, and more particularly, to a transfer device (apparatus) and a transfer member used in the image forming apparatus.
2. Related Background Art
In recent years, the development of image forming apparatuses such as printers, copying machines, and facsimile machines capable of operating at higher speed, having an increased number of functions and capable of forming images in an increased number of colors has progressed. Such image forming apparatuses heretofore proposed use various image forming systems. Among them, inline-type image forming apparatuses in which image forming means using a plurality of different colors are arranged in a row to successively transfer toner images directly onto a belt or onto a transfer material born by a transferring belt in a multiple transfer manner such that toner images are superposed one on another are considered to go mainstream as a color image forming apparatus in future because they have high operating speed and are capable of forming a multicolor image.
Inline-type image forming apparatuses are divided into a type (intermediate transfer type) in which toner images are temporarily transferred onto an intermediate transfer member in a multiple transfer manner and are thereafter transferred to a transfer material, and another type (transferring belt type) in which a transfer material is attracted to a transferring belt and multiple transfer is performed directly on this transfer material. From the viewpoint of reducing the size and manufacturing cost of the apparatus, it can be said that apparatuses of the latter type having a smaller number of system constituent elements are more advantageous.
Transferring belt type image forming apparatuses require a process in which, in the case of four-full-color image forming, transfer to an object such as a transfer material (e.g., paper or a transparent resin film) or a transferring belt having an instability factor with respect to the resistance value is performed four times and therefore have the disadvantage of being not sufficiently stable with respect to the kind of transfer material and conditions in environments where they are installed. Also, they require a considerably high transferring voltage for causing a sufficiently large transfer current at the time of automatic double-side image forming including repeating an image forming operation on a transfer material after the resistance of the transfer material has been increased by evaporation of water added at the time of fixation previously performed, or in an overhead transparency (OHT) mode in which image forming is performed on a transparent resin film which is insulating in the thickness direction.
Also, transfer-belt-type image forming apparatuses have four image forming stations (hereinafter referred to briefly as “station”) arranged between a position on the upstream side in the transfer material conveyance direction and a position on the downstream side in this direction. When the stations perform transfer successively from the upstream side, the transfer material or the transferring belt is charged up by receiving transfer charge, so that the transferring voltage required by the stations is increased with the distance from the upstream end position.
In a transferring part, discharge is caused between a photosensitive drum, a transfer material, a transferring belt, and a transfer member, e.g., (a transferring roller) to effect toner transfer and to move charge to the transfer material. If the transferring voltage is excessively high, excessive discharge or abnormal discharge occurs between the members, to which the high voltage is applied, resulting in failure to suitably transfer toner. In particular, if a leak site or the like exists between the transfer member and the transferring belt, discharge is concentrated at the leak site to cause deterioration in transferring performance.
Further, in a reversal developing system, variation in potential contrast between a dark portion potential and a light portion potential on an organic photo-conductive (OPC) material as seen from a transfer member leads to a change in amount of charge given to the OPC. A dark portion potential portion having a high transfer contrast is given transfer charge of a polarity opposite to that of the OPC charge potential, so that charging failure due to limitation of the increase in OPC potential at the time of subsequent image forming or OPC ghost is liable to occur frequently.
To ensure sufficiently high conductivity of a transferring roller, an electron-conductive filler such as carbon black or a metal oxide may be dispersed in a material for the transferring roller. In such a case, however, local resistance nonuniformity can occur easily due to nonuniformity of dispersion of the filler. In particular, a portion where the filler condenses becomes a leak site, at which a current is concentrated and can flow easily if the transferring voltage is high. The conduction mechanism in an electron-conductive material is such that electrons move between filler particles while hopping by the tunnel effect, and the current can be abruptly caused to flow readily and the resistance value becomes small when the applied voltage is increased.
To solve this problem, a method has been proposed in which an ion-conductive material whose resistance with respect to the applied voltage is comparatively stable and which has reduced local resistance nonuniformity is used to form a transfer member.
FIG. 4 shows voltage dependence of the resistance of an electronic conductor type of transferring roller and the resistance of an ionic conductor type of transferring roller adjusted so as to be equal to each other when the applied voltage is 500 V. In the region above 500 V, the resistance of the electronic conductor type of roller is lower than that of the ionic conductor type of roller. FIG. 5 shows the transfer efficiencies in uses of the two types of transferring rollers. It can be understood from FIG. 5 that the region through which the transferring efficiency is sufficiently high is narrow in the case of the electronic conductor type roller, while that in the case of the ionic conductor type roller is wider.
Generally, the transferring efficiency is improved if the transferring voltage (transferring bias) is increased. However, the electronic conductor type roller has a larger resistance value when the transferring voltage is low (e.g., lower than 500 V indicated in FIG. 4) and therefore the rate of increase in transferring efficiency in the case of use of the electronic conductor type roller is lower than that in the case of use of the ionic conductor type roller. That is, the transferring efficiency is peaked at a lower transferring voltage in the case of the ionic conductor type roller. If the transferring potential is excessively high, a toner transfer failure due to abnormal discharge or the like occurs and transferring efficiency decreases. Under such a condition, the reduction in transferring efficiency in the case of electronic conductor type roller starts before that in the case of the ionic conductor type roller. Since the electronic conductor type roller has a leak site due to resistance nonuniformity, it is thought that the transferring voltage at which abnormal discharge starts in the electronic conductor type roller is lower than the transferring voltage at which abnormal discharge starts in the ionic conductor type roller.
The ionic conductor type roller has a linear current-voltage characteristic as shown in FIG. 4 and is, therefore, substantially free from the problem relating to a high-transfer-contrast dark portion potential and the problem that an excessively large amount of current flows through a small-size non-paper-passing portion. Therefore, if this type of transferring roller is used in a reversal developing system, OPC ghost and charging failure do not occur easily.
With the ionic conductor type of transferring roller having the advantage that variation in resistance with respect to the applied voltage is small and that local resistance nonuniformity is also low, there arises a problem in that the resistance increases about ten times higher (at a rate of about one digit of magnitude in terms of common logarithm) to cause a reduction in transferring efficiency by a paper feeding durability test which is performed by feeding many paper (hereinafter referred to as “durability test”). FIG. 2 shows the transferring efficiency of an ionic conductor type of transferring roller before a durability test (in an initial state) and the transferring efficiency after the durability test (after its characteristics had been changed by the durability test). The high-transferring-efficiency region of the transferring roller after the durability test was reduced in comparison with that before the durability test. This transferring roller was tested after grinding away a roller surface portion by a thickness of about 500 μm therefrom. The result showed that the same resistivity characteristic as that before the durability test was again exhibited and the transferring efficiency was also improved. From this result, it can be thought that the transferring roller surface portion deteriorated by discharge or the like after the durability test to increase the resistance, and that, in a region of a lower transferring voltage, the rate of increase in transferring efficiency was reduced due to the increase in resistance. With respect to the observed increase in roller surface potion resistance, it is proper to think that not the uniform formation of a high-resistance layer but resistance nonuniformity occurred, and that abnormal discharge occurred earlier in a region of a higher transferring voltage to cause deterioration in transferring efficiency in comparison with the state before the durability test.